What Is the Incubation Period for the Flu?

You may be able to pass on the virus, even if you don't feel sick yet.

One of the most worrisome things about the flu—aside from the numerous symptoms—is that you can walk around with the virus without being aware that you're infected. Unfortunately, that also means you can spread influenza to plenty of other people during this time without knowing you have the flu in the first place.

It's known as the flu's incubation period, and "it's a big reason why the flu is so easily spread," Kathryn Boling, MD, a primary care physician at Baltimore's Mercy Medical Center, told Health. But while you've likely heard all about symptoms of the flu (fever, cough, fatigue, etc.), best prevention tactics (the flu shot), and even any possible remedies (Tamiflu), you might be a little less sure about what the incubation period of the flu is, what that means, and if there's any way to know if you're in it. Here's what experts want you to know.

What Is the Flu?

Influenza, more commonly known as the flu, is a contagious respiratory infection caused by several flu viruses that infect the nose, throat, and lungs, according to the National Institute of Allergy and Infectious Diseases (NIAID). The flu can be transmitted from person to person through the air via coughing or sneezing or by touching surfaces where there may be live flu viruses, according to Johns Hopkins Medicine.

Each year, the flu makes millions of people sick and causes thousands of hospitalizations and flu-related deaths. Anyone can get the flu and spread it to others—even healthy people, the NIAID says. The virus can be especially risky for certain groups, including the very young, elderly, pregnant women, and people with certain medical conditions.

People who get infected with the flu may develop symptoms like fever, chills, muscle aches, coughing, congestion, headache, and fatigue for about a week. Of note, since symptoms of the flu can also be the symptoms of other illnesses, you should see your healthcare provider to get confirmation of a flu diagnosis, per Johns Hopkins Medicine.

Most patients with the flu get better within two weeks, but some people can develop serious complications, like pneumonia, the NIAID notes. The Centers for Disease Control and Prevention (CDC) also indicate other possible flu-related complications in moderate to severe cases, which include sinus and ear infections; inflammation of different organs or muscle tissues; and multi-organ failures.

Furthermore, Johns Hopkins Medicine says that you should contact your healthcare provider if your symptoms worsen or if new symptoms occur while you have the flu.

What Is an Incubation Period?

It's important to point out that an incubation period isn't unique to the flu. COVID-19, for example, also has an incubation period—as do plenty of other infectious diseases.

"An incubation period is basically the timeframe from when you are exposed to some kind of pathogen to when you start showing signs or symptoms of that disease," Erik Blutinger, MD, an emergency medicine physician at Mount Sinai Queens, told Health. So, if you pick up the flu from a friend who is sick, the time between when you hung out and when you start showing symptoms is the incubation period.

However, your body begins mounting a response to the virus as soon as you're exposed to it. Per MedlinePlus, this happens through something called your innate immune system, or the one you were born with that helps keep harmful materials from entering your body. Once the virus gets past the innate immune system and has had time to multiply in your body, the more targeted adaptive immune system you have kicks in, and your symptoms ramp up.

How Long Is the Incubation Period of the Flu?

There's no exact time frame that's the same for every person with the flu. In general, the CDC says that the incubation period of the flu can range between one and four days, with the average length of time being two days.

Once people with the flu are showing symptoms, they are most contagious three to four days after illness begins, per the CDC. This time period can coincide with the incubation period. Most healthy adults may be able to infect others starting the day before they show symptoms and continue to be infectious up to seven days after they become sick, the CDC adds.

"People have the flu and are out and about doing their regular stuff, coming into contact with others before they get sick," Dr. Boling said. "You can infect plenty of people during that time."

Will You Know if You're in the Incubation Period for the Flu?

Unfortunately, you won't. "Generally, the flu is only recognized once symptoms develop," Dr. Blutinger said. "The incubation period isn't easily detected."

Hindsight really is everything, Dr. Boling said: Once you develop symptoms of the flu, you have at least some clue of when your incubation period was.

The only way to really have a tip-off of your incubation period is when you're actually living through it after having been exposed to someone with the flu, Dr. Boling said. But, unfortunately, most people are infected without knowing it.

Still, whether you figure out your incubation period or not, the proven methods to prevent the spread of COVID-19 also work to lower your risk of both contracting and passing on the flu. So, if you happen to catch the flu, wearing a mask while you're out and about, practicing social distancing, and ensuring that you receive your annual flu shot—if you're eligible to do so—should go a long way toward making sure your incubation period doesn't affect anyone else.

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